A DC system capable of efficiently transporting electric power with less power losses is attracting attention. The DC system is constructed of a DC cable, at least one terminal converter interconnected to a terminal of the DC cable, a load facility interconnected to an arbitrary location of the DC cable, a power generation facility and the like. Here, as the terminal converter, a DC/DC converter mutually interchanging electric power with a neighboring DC system of another voltage class or an AC/DC converter mutually interchanging electric power with a neighboring AC system.
Each terminal converter properly maintains the voltage in the DC system and also receives power from another neighboring system when the power value of the whole DC system obtained by subtracting the generated power value of the power generation facility from the power consumption value of the load facility is a positive value, that is, when there is a power shortage in the DC system or sends power to another neighboring system when the power value of the whole DC system is a negative value, that is, when there is a power surplus in the DC system to always maintain the balance of demand and supply in the DC system.
Conventionally, a control method of a terminal converter has been disclosed (see, for example, Patent Document 1). In Patent Document 1, one terminal converter among a plurality of terminal converters plays a role of maintaining the voltage in the DC system by performing constant voltage control, and the other terminal converters perform constant current control to avoid the concentration of the burden of power interchange on the terminal converter performing constant voltage control.
When there is a load facility or a power generation facility in the DC system, underpower and surplus power of the whole DC system increase and decrease so that each terminal converter needs to increase or decrease the interchange power according to the increase or decrease. Thus, a method to make coordination possible throughout the DC system even by autonomous control has been extensively studied by setting the V-I characteristic to each terminal converter, determining the output voltage value corresponding to its own output current when each terminal converter performs constant voltage control according to the V-I characteristic, and determining the output current according to its own connection point voltage when each terminal converter performs constant current control (see, for example, Non-Patent Document 1).
However, in the autonomous control method in which each terminal converter controls the DC system, each terminal converter is controlled only by its own information and thus, there is no guarantee that the voltage is maintained within the proper range at all points in the DC system. As a measure against such a problem, a centralized supervisory control method that collectively monitors and controls a DC system in a centralized manner is known (see, for example, Patent Document 2).
In Patent Document 2, a current transformer (CT) as a current sensor and a potential transformer (PT) as a voltage sensor are installed in the vicinity of a terminal converter, a load facility, and a power generation facility, and a terminal measuring device receiving the current value and the voltage value from each sensor calculates each power value and transmits the power value to a central monitoring control device. In the central monitoring control device, the output power value of the terminal converter, the power consumption value of the load facility, and the generated power value of the power generation facility in the DC system are collected, the highest voltage point and the lowest voltage point in the DC system are determined by the load-flow calculation in which the current value flowing at each point of the DC cable and the voltage drop and the voltage rise caused by a current flowing according to the Ohm's law (more specifically, width of voltage drop=current×line resistance) are determined, and the output voltage value and the output current value are determined so that both fall within the appropriate range of the voltage.